Millivolt power harvesting FET controller

ABSTRACT

Circuits and methods for controlling a transistor that has first, second and third terminals, wherein a voltage level at said first terminal controls in part a current flow from said second terminal to said third terminal. A controller receives an voltage existing across the second and third terminals of the transistor, generates an isolated voltage and uses that voltage to power components of the controller. The controller provides a voltage to the first terminal of the transistor, whereby the controller regulates the voltage across the second and third terminals of the transistor by regulating the voltage provided to the first terminal.

PRIORITY CLAIM

This application is a continuation-in-part of U.S. patent applicationSer. No. 15/221,328 filed Jul. 27, 2016, which claims priority under 35U.S.C. 119(e) to U.S. Provisional Patent Application 62/199,804,entitled “Millivolt Power Harvesting FET Controller,” filed Jul. 31,2015, the entire contents of which are hereby expressly incorporatedherein by reference.

BACKGROUND

Battery powered devices often include electronic safeguards to preventdamage to the internal electronics in the event of reverse batteryinstallation, accidental short circuiting, or other inappropriateoperation. Such electronic safeguards ensure that any reverse currentflow and reverse bias voltage is low enough to prevent damage to eitherthe battery itself or the device's internal electronics. One front-endcomponent that manufacturers often employ to effect reverse batteryprotection is the series diode. In higher power systems (e.g., two ampsor higher), the voltage drop across a series diode can cause excessiveamounts of power dissipation. More recently, field-effect transistorshave been employed to implement reverse battery protection. The mostrecent metal-oxide-semiconductor field-effect transistors (MOSFETs) havevery low resistance and are therefore ideal for providing reversecurrent protection with minimal loss. However, the use of a FET toprotect the battery during reverse current situations involves the useof a controller to regulate the voltage drop across the FET, and thecontroller itself consumes marginal power.

SUMMARY

An illustrative aspect of this disclosure is directed to a circuit thatincludes a transistor and a controller. The transistor has first, secondand third terminals, wherein a voltage level at said first terminalcontrols in part a current flow from said second terminal to said thirdterminal. The controller receives a voltage existing across the secondand third terminals of the transistor and uses that voltage to powercomponents of the controller. The controller provides a voltage to thefirst terminal of the transistor, whereby the controller regulates thevoltage across the second and third terminals of the transistor byregulating the voltage provided to the first terminal.

Another illustrative aspect of this disclosure is directed to a circuitthat includes a transistor and a controller. The transistor has first,second and third terminals, wherein a voltage level at said firstterminal controls in part a current flow from said second terminal tosaid third terminal. The controller includes a voltage converter and acontrol circuit. The voltage converter receives a voltage existingacross the second and third terminals of the transistor and convertsthat voltage to a power supply voltage that is a higher voltage than thereceived voltage. The control circuit is powered by the power supplyvoltage. The control circuit receives the voltage existing across thesecond and third terminals of the transistor and provides a controlvoltage to the first terminal of the transistor. The control circuitregulates the voltage across the second and third terminals of thetransistor by regulating the control voltage provided to the firstterminal.

Another illustrative aspect of this disclosure is directed to a methodof controlling a transistor comprising first, second and thirdterminals, wherein a voltage level at said first terminal controls inpart a current flow from said second terminal to said third terminal.Pursuant to said method, a voltage existing across the second and thirdterminals of the transistor is received. The voltage existing across thesecond and third terminals of the transistor is used to power a controlcircuit. The control circuit generates a control voltage based on thevoltage existing across the second and third terminals of thetransistor. The control circuit provides the control voltage to thefirst terminal of the transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of a circuit that includes abattery powering a load and that employs a transistor serving asreverse-current protection.

FIG. 2 is a schematic circuit diagram of an NFET transistor and anassociated controller.

FIG. 3 is a flow chart representing a method of controlling atransistor.

FIG. 4 is the same circuit shown in FIG. 1, except that the battery andload are isolated from ground.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings,wherein like reference numerals are used to designate similar orequivalent elements. Illustrated ordering of acts or events should notbe considered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

Illustrative aspects of the present disclosure are directed generallytoward powering a transistor controller using the voltage existingacross the terminals of the transistor being controlled. This generalconcept of the present disclosure is described herein with respect tocontrolling a series protection FET. It is to be appreciated, though,that aspects of the present disclosure can be applied to and implementedin a wide variety of applications.

FIG. 1 is a schematic circuit diagram of a circuit 100 that includes abattery 110 powering a load 120 and that employs a transistor 130serving as reverse-current protection. FIG. 1 shows an n-channel FET(NFET) 130 in the power path of the circuit 100, but it is to beappreciated that a p-channel FET could be employed instead of the NFET,with corresponding changes in circuit operation as will be appreciatedby those of skill in the art. The reverse-current protection transistor130 can also be implemented with other transistor types besidesfield-effect transistors. In circuit 100, the body diode 135 of NFET 130is oriented in the direction of normal current flow. The NFET 130 allowscurrent flow in the forward direction, i.e., from the source 140 to thedrain 145 of NFET 130 (i.e., from the battery 110 to the load 120), andblocks current in the reverse direction, i.e., from the drain 145 to thesource 140 of NFET 130. A controller 160 receives inputs from the source140 and the drain 145 of the NFET 130. The controller 160 generates acontrol signal, or gate driver signal, based on the drain-to-sourcevoltage V_(ds). The controller 160 provides the gate driver signal tothe gate 150 of the NFET 130. In this way, the controller 160 regulatesthe drain-to-source voltage V_(ds) in a control loop.

FIG. 4 is a schematic circuit diagram of a circuit 400 that includes abattery 410 powering a load 420 and that employs a transistor 430serving as reverse-current protection. FIG. 4 shows an n-channel FET(NFET) 430 in the power path of the circuit 400, but it is to beappreciated that a p-channel FET could be employed instead of the NFET,with corresponding changes in circuit operation as will be appreciatedby those of skill in the art. The reverse-current protection transistor430 can also be implemented with other transistor types besidesfield-effect transistors. In circuit 400, the body diode 435 of NFET 430is oriented in the direction of normal current flow. The NFET 430 allowscurrent flow in the forward direction, i.e., from the source 440 to thedrain 445 of NFET 430 (i.e., from the battery 410 to the load 420), andblocks current in the reverse direction, i.e., from the drain 445 to thesource 440 of NFET 430. A controller 460 receives inputs from the source440 and the drain 445 of the NFET 430. The controller 460 generates acontrol signal, or gate driver signal, based on the drain-to-sourcevoltage Vds. The controller 460 provides the gate driver signal to thegate 450 of the NFET 430. In this way, the controller 460 regulates thedrain-to-source voltage Vds in a control loop.

The difference between the circuits of FIGS. 1 and 4, is that in FIG. 4,ground is not utilized for the return path of current from the load 420to the battery 410.

In an illustrative aspect of the present disclosure, the controller 160also receives its power from the voltage drop across the NFET 130, i.e.,from the drain-to-source voltage V_(ds).

FIG. 2 is a schematic circuit diagram of an NFET transistor 230 and anassociated controller 260. In FIG. 2, NFET 230 corresponds to NFET 130of FIG. 1, and controller 260 corresponds to controller 160 of FIG. 1.The controller 260 includes a voltage conversion circuit 265, anoscillator 270, a reservoir capacitor 275, a differential amplifier 280,and an offset voltage source 285. The controller 260 receives its powerfrom the voltage drop across the NFET 230, i.e., from thedrain-to-source voltage V_(ds). In an illustrative embodiment, thecontroller 260 receives all of its power from the drain-to-sourcevoltage V_(ds), without receiving any additional power from other powersources. The voltage conversion circuit 265 receives the drain-to-sourcevoltage V_(ds) of the NFET 230 and converts it to a voltage levelappropriate for powering components of the controller, such asdifferential amplifier 280. In certain illustrative embodiments of thepresent disclosure, the voltage converter 265 comprises a capacitivecharge pump circuit, while in other illustrative embodiments, thevoltage conversion circuit 265 comprises an inductive voltage boostcircuit. The oscillator 270 receives the drain-to-source voltage V_(ds)of the NFET 230, generates an oscillating signal, and provides theoscillating signal to the voltage conversion circuit 265 to drive thecharge pump circuit. The oscillating signal generated by the oscillator270 can be a square wave, a pulsed DC signal, or any other type ofoscillating signal. The reservoir capacitor 275 stores the supplyvoltage generated by the voltage conversion circuit 265. The reservoircapacitor 275 can be either internal to the controller 260 or can beexternal. The controller 260 can be implemented in some cases withoutthe reservoir capacitor. The supply voltage generated by the voltageconverter 265 is used to power a control circuit, represented in FIG. 2by the differential amplifier 280 and the offset voltage source 285,that regulates the drain-to-source voltage V_(ds) of the NFET 230 bycontrolling the voltage of a control signal provided to the gate 250 ofthe NFET 230 in a control loop based on said drain-to-source voltageV_(ds).

The voltage generated by the voltage converter 265 used to power thedifferential amplifier 280 is isolated from ground, whether or not thereservoir capacitor 275 is utilized. In the illustrative embodiment ofFIG. 2, the voltage of the offset voltage source 285 is added to thesource voltage of the NFET 230 and the summed voltage signal is providedto the non-inverting input of the differential amplifier 280. Thevoltage level of the offset voltage source 285 corresponds to a targetvoltage level of the drain-to-source voltage V_(ds). The drain voltageof the NFET 230 is provided to the inverting input of the differentialamplifier 280. The output of the differential amplifier 280 is providedto the gate 250 of the NFET 230. In this way, the control circuitrepresented by the differential amplifier 280 and the offset voltagesource 285 works to maintain the drain-to-source voltage V_(ds) at avoltage level substantially corresponding to the target voltagerepresented by the offset voltage source 285.

In operation, at startup, because of the body diode orientation of theN-channel FET 230, there will be initial current flowing through thebody diode 235 if the load 120 is on. Therefore, even if the controller260 is not powered, this will not interrupt the flow of current.However, as the current starts to flow through the body diode 235, avoltage will be generated across the source 240 and drain 245 of theNFET 230, specifically the voltage of a PN junction. This voltage canvary from NFET to NFET, and across temperatures, but nominally may bearound 0.5V-0.7V. This voltage is also seen by the controller 260,across the source terminal 240 and the drain terminal 245 of the NFET230. Using this voltage, the controller 260 will have enough voltage toturn on and startup its own circuitry, namely the voltage conversioncircuit 265 and the oscillator 270.

The voltage conversion circuit 265 uses the body voltage, i.e., thedrain-to-source voltage V_(ds) (0.5V-0.7V) to generate an internalsupply voltage. In the illustrative embodiment of FIG. 2, this internalsupply voltage is shown to be approximately 5V, but this voltage levelis merely illustrative and other internal supply voltage levels can beused. In an illustrative embodiment, the voltage converter 265 generatesanywhere from approximately 0.5 mWatts to 10 mWatts. The controller 260then enters the regulation mode, wherein it varies the gate voltage ofthe NFET 230 in a controlled loop to keep the drain-to-source voltageV_(ds) at the desired target level. In the illustrative embodiment ofFIG. 2, this target voltage is shown to be 50 mV, but this voltage levelis merely illustrative and other target V_(ds) levels can be used. In anillustrative embodiment, the controller 260 maintains the gate voltageat a level that is just high enough to give sufficient V_(ds) to keepthe voltage conversion circuit 265 running and generating the internalsupply voltage (5V). As mentioned, this internal supply voltage is usedto power the differential amplifier 280 and/or other control circuitry.

The regulated source-to-drain voltage V_(ds) of the NFET 230 isdependent on the threshold voltage (V_(threshold)) of the NFET deviceused. Therefore, the regulation self-adjusts when V_(threshold)fluctuates across temperature by varying the gate voltage to match theV_(ds).

In the illustrative embodiment of FIG. 2, the controller 260 is shown toinclude a single voltage conversion circuit 265. In an alternativeembodiment, the controller 260 may include two separate voltageconversion circuits in order to accommodate the two different V_(ds)levels that exist at start-up and in the regulation mode. For example,the controller 260 can include a first voltage conversion circuitconfigured to receive a V_(ds) of 0.5V-0.7V at start-up and convert saidV_(ds) to the level of the internal supply voltage (e.g., 5V), and asecond voltage conversion circuit configured to receive a V_(ds) ofapproximately 50 mV in the regulation mode and convert that V_(ds) tothe level of the internal supply voltage. Alternatively a single voltageconversion circuit 265 can be figured to operate at the V_(ds) levels ofboth the start-up and regulation modes. Similarly, while FIG. 2 shows asingle oscillator 270, in an alternative embodiment the controller 260can include two separate oscillators in order to accommodate the twodifferent V_(ds) levels that exist at start-up and in the regulationmode.

FIG. 3 is a flow chart representing a method of controlling a transistorcomprising first, second and third terminals, wherein a voltage level atsaid first terminal controls in part a current flow from said secondterminal to said third terminal. At step 300, a voltage existing acrossthe second and third terminals of the transistor is received. At step310, the voltage existing across the second and third terminals of thetransistor is used to power a control circuit. At step 320, the controlcircuit generates a control voltage based on the voltage existing acrossthe second and third terminals of the transistor. At step 330, thecontrol circuit provides the control voltage to the first terminal ofthe transistor.

It is noted that the embodiments disclosed herein are illustrativerather than limiting in nature and that a wide range of variations,modifications, changes, and substitutions are contemplated in theforegoing disclosure. Furthermore, in some instances, some features maybe employed without a corresponding use of the other features.Accordingly, it is appropriate that the appended claims be construedbroadly and in a manner consistent with the broad inventive conceptsdisclosed herein.

What is claimed is:
 1. A circuit providing reverse current protectioncomprising: a transistor comprising first, second and third terminals,wherein a voltage level at said first terminal controls a current flowfrom said second terminal to said third terminal; and a voltageconverter configured to receive a voltage existing across the second andthird terminals of the transistor and generating a voltage to powercomponents of a controller, the controller being coupled to a voltageexisting at the second and third terminals and isolated from a commonground by passive components and being configured to provide a voltageto the first terminal of the transistor, whereby the controller isoperable to regulate the voltage across the second and third terminalsof the transistor by regulating the voltage provided to the firstterminal.
 2. The circuit of claim 1 wherein the received voltageexisting across the second and third terminals of the transistor is thecontroller's sole source of power.
 3. The circuit of claim 1 wherein thecontroller is operable to maintain the voltage across the second andthird terminals of the transistor at a predetermined target voltage byregulating the voltage provided to the first terminal of the transistor.4. The circuit of claim 3 wherein said predetermined target voltagecorresponds to a minimum voltage required to power the components of thecontroller.
 5. The circuit of claim 1 wherein the transistor comprises afield-effect transistor (FET) and wherein said first terminal comprisesthe gate of the FET.
 6. The circuit of claim 5 wherein the transistorcomprises an n-channel FET (NFET) and wherein said second terminalcomprises the source of the NFET, and said third terminal comprises thedrain of the NFET.
 7. The circuit of claim 1 further comprising abattery powering a load, the battery and the load being isolated fromground.
 8. A circuit comprising: a transistor comprising first, secondand third terminals, wherein a voltage level at said first terminalcontrols a current flow from said second terminal to said thirdterminal; and a controller comprising: a voltage converter configured toreceive a voltage existing across the second and third terminals of thetransistor and convert said voltage to a power supply voltage that is ahigher voltage than said received voltage and isolated from a commonground; and a control circuit powered by said power supply voltage andconfigured to receive the voltage existing across the second and thirdterminals of the transistor and to provide a control voltage to thefirst terminal of the transistor, whereby the control circuit isoperable to regulate the voltage across the second and third terminalsof the transistor by regulating the control voltage provided to thefirst terminal.
 9. The circuit of claim 8 wherein said voltage convertercomprises a charge pump circuit configured to receive a voltage existingacross the second and third terminals of the transistor and convert saidvoltage to a power supply voltage that is a higher voltage than saidreceived voltage.
 10. The circuit of claim 9 wherein the controllerfurther comprises an oscillator configured to receive a voltage existingacross the second and third terminals of the transistor and to generatean oscillating signal, and to provide the oscillating signal to thecharge pump to drive the charge pump circuit.
 11. The circuit of claim 9wherein the controller further comprises a reservoir capacitor coupledto receive and store the power supply voltage generated by the chargepump circuit.
 12. The circuit of claim 8 wherein the control circuit isoperable to maintain the voltage across the second and third terminalsof the transistor at a predetermined target voltage by regulating thevoltage provided to the first terminal of the transistor.
 13. Thecircuit of claim 12 wherein the control circuit comprises: adifferential amplifier powered by the power supply voltage and having aninverting input coupled to the third terminal of the transistor andhaving an output coupled to the first terminal of the transistor; and anoffset voltage source coupled between the second terminal of thetransistor and a non-inverting input of the differential amplifier, thevoltage of the offset voltage source corresponding to the target voltageacross the second and third terminals of the transistor.
 14. The circuitof claim 12 wherein said predetermined target voltage corresponds to aminimum voltage required to power the control circuit.
 15. The circuitof claim 8 wherein the received voltage existing across the second andthird terminals of the transistor is the control circuit's sole sourceof power.
 16. The circuit of claim 8 wherein the transistor comprises afield-effect transistor (FET) and wherein said first terminal comprisesthe gate of the FET.
 17. The circuit of claim 16 wherein the transistorcomprises an n-channel FET (NFET) and wherein said second terminalcomprises the source of the NFET, and said third terminal comprises thedrain of the NFET.
 18. A method of controlling a transistor providingreverse current protection, the transistor comprising first, second andthird terminals, wherein a voltage level at said first terminal controlscreate a current flow from said second terminal to said third terminal,the method comprising: receiving, at a voltage converter, a voltageexisting across the second and third terminals of the transistor;powering a control circuit with an output of the voltage converter;using said voltage existing across the second and third terminals of thetransistor to power the control circuit using voltage isolated from acommon ground; generating, with the control circuit, a control voltagebased on the voltage existing across the second and third terminals ofthe transistor; and providing, with the control circuit, the controlvoltage to the first terminal of the transistor.
 19. The method of claim18 wherein said generating a control voltage comprises generating acontrol voltage such that the voltage across the second and thirdterminals of the transistor is substantially maintained at apredetermined target voltage.
 20. The circuit of claim 19 wherein saidpredetermined target voltage corresponds to a minimum voltage requiredto power the control circuit.
 21. The method of claim 18 wherein thereceived voltage existing across the second and third terminals of thetransistor is the control circuit's sole source of power.